The present invention relates to a process for the production of conductive surface coatings using a dispersion containing electrostatically stabilised silver nanoparticles, to dispersions particularly suitable for this process, and to a process for their preparation.
In Adv. Mater., 2003, 15, No. 9, 695-699, Xia et al. describe the preparation of stable aqueous dispersions of silver nanoparticles with poly(vinyl-pyrrolidone) (PVP) and sodium citrate as stabilisers. Xia thus obtains monodisperse dispersions containing silver nanoparticles having particle sizes of less than 10 nm and a narrow particle size distribution. The use of PVP as polymeric stabiliser results in steric stabilisation of the nanoparticles against aggregation. However, such steric polymeric dispersion stabilisers have the disadvantage that, in the resulting conductive coatings, because of the surface coating on the silver particles, they reduce the direct contact of the particles with one another and accordingly the conductivity of the coating. According to Xia it is not possible to obtain such stable monodisperse dispersions without using PVP.
EP 1 493 780 A1 describes the production of conductive surface coatings using a liquid conductive composition of a binder and silver particles, wherein the above-mentioned silver-containing silver particles can be silver oxide particles, silver carbonate particles or silver acetate particles, which in each case can have a size of from 10 nm to 10 μm. The binder is a polyvalent phenol compound or one of various resins, that is to say in any case a polymeric component. According to EP 1 493 780 A1, a conductive layer is obtained from this composition after application to a surface with heating, whereby heating is preferably to be carried out at temperatures of from 140° C. to 200° C. The conductive compositions described according to EP 1 493 780 A1 are dispersions in a dispersant selected from alcohols, such as methanol, ethanol and propanol, isophorones, terpineols, triethylene glycol monobutyl ethers and ethylene glycol monobutyl ether acetate. EP 1 493 780 A1 again mentions that the silver-containing particles in the dispersant are preferably to be protected against aggregation by addition of dispersion stabilisers such as hydroxypropylcellulose, polyvinylpyrrolidone and polyvinyl alcohol. These dispersion stabilisers are also polymeric components. The silver-containing particles are accordingly always sterically stabilised against aggregation in the dispersant by the above-mentioned dispersion stabilisers or the binder as dispersion stabiliser. However, such polymeric dispersion stabilisers with a steric action have the disadvantage—as already mentioned above—that, in the resulting conductive coatings, because of the surface coating on the silver particles, they reduce direct contact of the particles with one another and accordingly the conductivity of the coating. Although the organic solvents used as dispersants in 1 493 780 A1 accelerate the drying time, or reduce the drying temperatures, of the coatings applied therewith, so that even temperature-sensitive plastics surfaces can be coated therewith, such organic dispersants attack or can diffuse into the surface of plastics substrates, which can lead to swelling or damage of the substrate surface and of any underlying layers.
US 2009/104437 A1 discloses a process for coating surfaces with conductive coatings by means of electrostatic self-assembling. However, coating is carried out by means of an expensive, time-consuming multi-stage dipping process.
WO 03/038002 A1 discloses an inkjet printer composition obtained by reducing silver nitrate with boron hydride or citrate. However, the composition is not stable and is accordingly not suitable for the production of surface coatings.
WO 2009/044389 A2, WO 2005/079353 A2, JOURNAL OF MATERIALS CHEMISTRY, Vol. 17, 2007, pages 2459-2464, JOURNAL OF PHYSICAL CHEMISTRY, AMERICAN CHEMICAL SOCIETY, Vol. 86; No. 17, pages 3391-3395 and JOURNAL OF PHYSICAL CHEMISTRY B, Vol. 103, pages 9533-9539 also disclose silver nanoparticles stabilised with citrates and dispersions of those silver nanoparticles. However, there is no indication in any of those documents as to how conductive surface coatings can be produced by means of such dispersions in a manner that is simple and kind to the substrate.
Accordingly, there continued to be a need for a process for coating surfaces with conductive coatings using dispersions containing silver nanoparticles, in which process it is possible to use short drying and sintering times and/or low drying and sintering temperatures, so that even temperature-sensitive plastics surfaces can be coated, but in which damage to such surfaces by the dispersant used is not to be feared, wherein in this process too, premature aggregation and accordingly flocculation of the silver nanoparticles in the dispersions used is to be prevented by suitable stabilisation.
Starting from the prior art, the object was therefore to find such a process and dispersions suitable therefor. The above-mentioned, disadvantageous combination of improved stabilisation against aggregation with reduced conductivity of the surface coatings produced from the dispersions is thereby to be avoided. In preferred embodiments, the possibility of using this process for the coating of plastics surfaces with short drying and sintering times and/or low drying and sintering temperatures is not to be accompanied by the risk of damage to the surfaces.